It has previously been proposed to provide an inflatable element or air bag in a motor vehicle in a position such that in the event of an accident involving a side impact or a roll-over incident, the inflated element is inflated so as to become disposed between the side of the vehicle and the head of a vehicle occupant in order to protect against the occupant's head coming into contact with or passing through a window in the side of the vehicle. Inflatable elements of this type are generally known as “inflatable curtains” or “side curtain airbags”.
It has been proposed to divide the inflatable element into a plurality of interconnected “cells” by forming the element from two woven layers of fabric with selected regions of the two fabric layers being woven together by a so-called one-piece weaving technique in order to interconnect the two layers and thereby define the periphery of each of the cells. The interwoven regions which define the periphery of each of the cells may be relatively large, thereby reducing the overall volume of the inflatable cells and the volume of gas which is required to inflate the element such that the element may be inflated more quickly. Dividing these inflatable elements into a plurality of cells prevents the element from “ballooning” when inflated, thereby enabling the inflated shape of the element to be controlled.
However, dividing the inflatable element into a plurality of cells by weaving together the two layers of fabric which form the element in selected regions also has certain disadvantages. It is not as easy to control the inflation of a large number of small cells as it is to control the inflation of one or two larger chambers. Thus, whilst the volume of gas required to inflate the element may be reduced, the regions where the two layers of fabric are woven together present obstructions to the natural flow of inflation gas into the inflatable element during inflation and these interwoven regions may therefore impede the inflating flow of gas and result in the element inflating in a manner which is not exactly as desired.
In order to address these problems it has therefore been proposed to prevent ballooning and to control the inflated shape of such inflatable elements by means other than dividing them into cells by a one-piece weaving technique. One such proposal is to use a so-called “tether weave” to provide a number of discrete tethers between the two opposed layers of fabric defining the inflatable element. An arrangement of this type is proposed in U.S. Pat. No. 6,296,276 in which internal tethers are defined within the inflatable element by threads which form an integral part of the weave of the fabric layers. Such a tether is defined by threads which depart from the weave of one of the two layers of fabric and extend towards and into the weave of the other layer of fabric at discrete points. The resulting inflatable element has a more open internal structure than the one-piece-woven multiple-cell type, with the threads serving as tethers between the two layers of fabric to prevent ballooning but without representing significant obstacles to the flow of inflating gas. However, it has been found that conventional “tether weave” arrangements of the type described above have their own problems. One such problem is that as the two fabric layers move apart from one another during inflation of the inflatable element, the threads defining the tethers are pulled through the weave of the two layers, thereby causing the inflatable element to contract in a longitudinal direction compared to its flat uninflected condition. In the case of a typical inflatable curtain arrangement, this contraction can be of the order of 25% of the uninflated length of the curtain. This is a problem which is more significant for tether weave arrangements than one-piece woven arrangements. For example, a conventional inflatable curtain of a one-piece woven construction for a motor car might contract in length by approximately 96 mm as it inflates, whereas a comparable curtain of conventional tether weave construction can contract by as much as 135 mm. As will be appreciated, this can significantly affect the coverage of the inflated curtain and hence its effectiveness.
The present invention seeks to provide an improved safety device of the type described above.